1. Field of the Invention
The present invention relates to a heat storage composition superior in long-term stability, which is composed mainly of sodium sulfate decahydrate. More particularly, the invention relates to a heat storage composition characterized by additionally containing calcium sulfate as a solid-liquid separation inhibitor.
2. Description of the Prior Art
There has long been known the concept that thermal energy is stored and reproduced by utilizing the absorption and release of heat due to change in phase of a hydrate salt compound between fusion and solidification. According to results of studies reported up to now, it is pointed out that two problems arise when this method for heat storage is reduced into practical use. The first problem is that hydrate salts often show a so-called supercooling phenomenon without undergoing the phase transition of fusion or solidification at the respective fusion points or freezing points. Accordingly such supercooling needs to be prevented in order to cause the phase transition at a definite temperature. For example, U.S. Pat. No. 2,677,664 describes a method for preventing such supercooling which comprises addition of borax (Na.sub.2 B.sub.4 O.sub.7 .multidot.10H.sub.2 O) as a supercooling inhibitor for Na.sub.2 SO.sub.4 .multidot.10H.sub.2 O. Crystals of Na.sub.2 B.sub.4 O.sub.7 .multidot.10H.sub.2 O are reported to be in epitaxial relation with crystals of Na.sub.2 SO.sub.4 .multidot.10H.sub.2 O and when contained in a supersaturated solution of Na.sub.2 SO.sub.4 .multidot.10H.sub.2 O, promote the crystallization of Na.sub.2 SO.sub.4 .multidot.10H.sub.2 O, thus preventing the supercooling effectively (Industrial and Engineering Chemistry, Vol. 44, pp. 1308-1310 (1952)). The use of Ba(OH).sub.2 or Ba(OH).sub.2 .multidot.8H.sub.2 O (Japanese Patent Publication No. 9596/1978) and the use of BaI.sub.2 or BaSO.sub.4 (Japanese Patent Application Laid-Open No. 102675/1980) are also known as methods for preventing the supercooling of CaCl.sub.2 .multidot.6H.sub.2 O.
The second problem is the occurrence of a solid-liquid separation phenomenon, that is, the sedimentation of an anhydrous salt during the phase transition of the hydrate salt thereof. For example, Na.sub.2 SO.sub.4 .multidot.10H.sub.2 O is decomposed at 32.4.degree. C. to form an anhydrous salt thereof and this anhydrous salt settles on the bottom. On cooling to a temperature below 32.4.degree. C., the anhydrous salt is recombined with water to become Na.sub.2 SO.sub.4 .multidot.10H.sub.2 O. However, crystals of Na.sub.2 SO.sub.4 .multidot.10H.sub.2 O cover the surface and hence the lower part of the anhydrous salt is inhibited from the water recombination, thus producing floating water, i.e., bleeding. Therefore, the anhydrous salt needs to be prevented from the sedimentation. In general a solid-liquid separation inhibitor is used for the purpose of preventing the sedimentation. In this case, the long-term stability of the resulting heat storage composition should be satisfied. The use of an organic material such as natural rubber or a synthetic high polymer as the solid-liquid separation inhibitor is undesirable since these organic materials can be gradually hydrolyzed or attacked by microorganisms to decomposition. The use of inorganic materials has been proposed such as a porous support (Japanese Patent Publication No. 6108/1978), a clay type of material having lath-like particles for providing a thixotropic property (Japanese Patent Application Laid-Open No. 34687/1978), and a superfine powder of anhydrous silicic acid (SiO.sub.2) (Chemical Week, Mar. 1, 1978, p. 34). However, the use of these inorganic materials tends to result in a gradual reduction of the viscosity of the heat storage composition and leads to the solid-liquid separation during repetition of the heat absorption-radiation cycle for a long time, so that the ability thereof to prevent the solid-liquid separation cannot be said to be sufficient. Additionally, some of these inorganic materials are natural products yielding in special teritories and some of these are expensive. Thus, economy becomes an issue when these inorganic materials are practically used, since the original object of heat storage materials is energy conservation.
In view of the present situation noted above, the present inventors made extensive studies on the long-term stability of heat storage materials composed mainly of Na.sub.2 SO.sub.4 .multidot.10H.sub.2 O, and as a result have found that the stability is markedly improved by addition of calcium sulfate as a solid-liquid separation inhibitor to Na.sub.2 SO.sub.4 .multidot.10H.sub.2 O. Based on this finding, the present invention has been accomplished.